Group III-N nitride compounds and their alloys have been developed for various optoelectronic and electronic applications. For example, GaN, AlN, AlGaN, InN, InGaN, InAlN and InGaAlN have physical and electronic properties that make them ideal candidates for various electronic and opto-electronic devices. These materials exhibit a direct band gap structure, high electric breakdown field, and high thermal conductivity, which make them suitable for devices including high brightness light emitting diodes (LEDs) with high internal quantum efficiency (IQE) and have a variety of important applications.
Additionally, ternary alloy compounds or materials such as InxAl1-xN and InxGa1-xN can be used to cover a wide range of band gap energies from about 0.7 eV (where x equals 1) to 6.2 eV (where x equals 0) and from about 0.7 to 3.4 eV, respectively. Quaternary compounds or materials, such as InxGayAl1-x-yN, are also of interest as certain compositions can provide lattice matched pairs for GaN and other binary and ternary nitride compounds for reducing defect densities in nitride based hetero-structures.
One method of growing such materials is metal organic chemical vapor deposition (MOCVD) techniques. With MOCVD, group III-V compounds are grown from the vapor phase using metal organic compounds as sources of the Group III metals. Trimethylindium (TMI) is typically used as an indium source material, trimethylaluminum (TMA) is used as an aluminum source material and trimethylgallium (TMG) is used as a gallium source material. Ammonia gas is typically used as a nitrogen source. Materials are supplied to a MOCVD reactor from external source tanks Inside the MOCVD reactor, a metal organic material source reacts with ammonia resulting in deposition of an epitaxial layer of a nitride material on a substrate. In order to control the electrical conductivity of grown materials, electrically active impurities are introduced into the reaction chamber during material growth. Undoped III-V compounds normally exhibit n-type conductivity. Donor impurities such as silicon or germanium are used to control n-type conductivity. Magnesium impurities are usually used in the form of metal organic compounds to control p-type conductivity.
MOCVD has been used to fabricate p-type III-N materials, and a variety of semiconductor devices employing both p-n and p-i-n junctions have been demonstrated including light emitting diodes (LEDs), laser diodes (LDs), photo-detectors, and transistors. High brightness LEDs and blue and ultraviolet (UV) lasers have been demonstrated based on group III nitride multi layer epitaxial structures including single or multiple quantum wells and quantum dots.
In many of these devices, indium-containing alloy materials including InGaN and InGaAlN are the main components of the active regions. For example, light emitting regions of high brightness blue LEDs and LDs are typically made from InGaN alloys, in particular, InGaN quantum well structures. Parameters of the devices can be controlled by changing alloy compositions. For example, for blue LEDs, the InN content in the InGaN light emitting region may be about 15 mol. %, whereas InN content of about 25 mol. % is required for green LEDs.
MOCVD has also been used to produce technology capable of producing multi-layer hetero-structures that are used for electronic or optoelectronic devices. Typical structures include AlGaN/GaN hetero-structures for high frequency transistors and AlGaN/InGaN/GaN pn hetero-structures for light emitting devices. An important elements of these structures are quantum well structures, which are hetero-structures having thicknesses of separate layers of several nanometers. In such structures, light emitting efficiency may be increased due to quantum effects taking place in nanometer thick epitaxial layers comprising hetero-junctions.
Although MOCVD has been used in the past, this fabrication method has a number of limitations. Significantly, grown materials suffer from high defect densities, poor conductivity or doping control, and non-uniformity. For example, green LEDs based on InGaN alloy materials with relatively high InN content (more than 20 mol. %) fabricated by MOCVD show low efficiency and output power, and nitride-based yellow and red LEDs require even higher InN content and currently are not commercially available due to poor performance. Such poor material characteristics limit the applicability of such materials, and the resulting structures may not be suitable for applications such as high speed communication THz electronics, solar sells, and advanced sensors. Further, success in controlling p-type conductivity in nitride materials is limited to use of GaN and to some extent, AlGaN and InGaN alloys with low AlN and InN content, respectively. Other limitations of MOCVD include high fabrication costs resulting from high costs source materials, low deposition rates and complicated growth procedures and apparatuses. MOCVD requires operations in a high vacuum environment and the associated pumping processes and machinery. Further, deposition rates achieved by MOCVD typically do not exceed one or two microns per hour, thus limiting the thickness of deposited materials and rendering MOCVD unsuitable for bulk growth.
It is also known to grow such materials using Molecular Beam Epitaxy (MBE). MBE, however, suffers many of the same shortcomings as MOCVD. For example, MBE involves high fabrication costs and operation in a vacuum environment. Further, MBE techniques also suffer from slow deposition rates, which limit the thicknesses of materials grown by MOCVD and limit or prohibit effective bulk growth.
Hydride vapor phase epitaxy (HVPE) has also been investigated for the fabrication of III-V nitride materials. With HVPE, gallium and aluminum metals are typically used as source materials for GaN and AlN HVPE growth, respectively. HVPE can be performed at atmospheric pressure, thus eliminating the need for vacuum processes and equipment of other systems. HVPE is also convenient for mass production of semiconductor materials and devices due to its low cost, excellent material characteristics, flexibility of growth conditions, and good reproducibility.
HVPE also offers advantages in material quality (low defect density), growth rate, controllable doping, process and equipment simplicity, and low fabrication cost. High deposition rates of about 200 microns per hour are characteristic for bulk growth of GaN and AlN materials with high crystalline quality. It is known that for nitride materials grown on foreign substrates (e.g. sapphire), defect densities rapidly decrease with layer thickness. The ability to deposit from 10 to 100 microns thick layers and subsequently reduce defect densities by orders of magnitude provides significant advantages over MOCVD and MBE.
Published results on HVPE growth of InGaN materials are limited, and limited data on HVPE growth of InGaAlN is available. InGaN layers grown using HVPE were demonstrated in 1997 by Takahashi et al. In this study, growth was performed from InCl3 and GaCl3 as initial group III materials sources and ammonia as the nitrogen source. All of the source materials were pre-synthesized outside of the reactor and placed inside the reactor without reaction with a reactive gas, such as HCl gas. Optical or structural properties of grown material were not reported.
In another study by Sato et al., HVPE growth of InGaN layers was performed using indium and gallium metals that were placed in a source zone in the same HCl flow channel. The gas resulting from the reaction of indium and gallium metals in the source zone was provided directly to a growth zone, and growth was carried out using ammonia. Growth temperatures ranged from 520° to 1010° C. Growth rates up to 1.5 microns/hour were observed. Grown InGaN layers had weak and broad photo luminescence (PL). A weak PL peak was observed at around 430-440 nm.
While growth of InGaN has been performed in the past, growth of device quality indium-containing alloy materials such as InGaN alloys has not been successfully demonstrated, e.g., due to impurities in externally generated or pre-synthesized source materials. For example, in one study, the smallest value of the full width at half maximum (FWHM) of x-ray rocking curves using prior HVPE methods is about 1482 arc seconds for the (00.2) InN peak. X-ray rocking curve data for the (10.2) InN reflection, which can be measured only for high crystal quality materials, was not reported.
Accordingly, it would be desirable to have HVPE reactors and growth methods capable of growing high quality, low defect density indium-containing nitride materials and structures, such as InGaN and InGaAlN. Further, it would be desirable to be able to grow such materials and structures throughout their composition ranges. Additionally, it would be desirable to grow high quality materials and structures with greater thicknesses than known systems. It would also be desirable to be able to generate and collect source materials inside a HVPE reactor so that source material properties are defined by initial materials in order to reduce or prevent source material contaminants resulting from generation in and/or introduction of source materials from an external environment.